“Mamma Mia!” is the blockbuster 1990s musical based on the chartbuster songs by 1970s Swedish pop group ABBA. Admit it: You’re singing already.

3-D Theatricals has taken on this show as its last production of this season. 3-D was certain this one would be its own box-office bonanza, following the less-populist but far more artistic “Parade” it produced two months ago. “Mamma Mia!” started selling fast.

But perhaps this sure-hit status has caused the show to be under-rehearsed and under-performed. It lacks the crispness of the others this season.

Sophina Brown, center, with the cast of “Mamma Mia!” (Caught in the Moment Photography)

Catherine Johnson’s book stitches together iconic ABBA songs, intersperses lesser-known ones and comes up with the story of Sophie (Flynn Hayward) on the eve of her wedding on a Greek island. She has grown up here, where her American mother, Donna (Sophina Brown), has established a taverna. But Sophie never knew who her father is.

Having pried into her mother’s diary a few months before the wedding, Sophie found three names of men who might be her dad. And though it’s the 1990s, she also found their addresses and invited them to the wedding, not cluing her mother in to her plans.

So here come those three men. American Sam (Martin Kildare) is now an architect; British Harry (Corky Loupe) is a banker; and Australian Bill (Michael Cavinder) is an outdoor adventurer and writer. Each thinks Donna invited him, and each thinks she wants to rekindle the romance they had in the ’70s.

Donna’s two good friends arrive for the wedding. Dramaturgically needed to reveal a bit of backstory, vocally needed for harmony on a few songs, coincidentally these two were the backup singers in their ’70s trio Donna and the Dynamos.

The wealthy, thrice-divorced Tanya (Janna Cardia) comes clad inappropriately for an island wedding but offers the best opportunities for costume designer Alexandra Johnson to splash out. Meanwhile, the zany, never-wed Rosie (Candi Milo) is satisfactorily comfy in her ill-fitting overalls.

David F.M. Vaughn directs. On opening night, too much seemed to be under-rehearsed, if ever staged. Most noticeably, Donna sings “The Winner Takes It All” to a stock-still Sam. Brown stands facing the audience for most of the song, turns toward Sam briefly, and then turns to the audience again. That’s it for staging this number, which should have so much emotion behind it as Donna reveals her broken heart to him.

Even vocally, this production disappoints. Cardia and Milo have the best voices on the stage, but no one captures the power and range of the original vocals, particularly in the lower parts of their ranges. So the audience may come to realize that the members of ABBA — Agnetha Fältskog, Björn Ulvaeus, Benny Andersson and Anni-Frid Lyngstad — and particularly the two women singing lead, were superb vocalists.

And that may be the happiest surprise of all. Who is Sophie’s biological father? Who does Donna still love? The ending of this story won’t be spoilered here.

The production was reviewed at the Redondo Beach Performing Arts Center. It moves to Cerritos Center for the Performing Arts, Aug. 17-26.